The invention relates to a fuse-element for electric fuses that is shaped as a metal strip with one or more zones of diminished cross-section.
It is a well-known tendency in this field to develop special types of electric fuses with different time/current characteristics chosen in a manner as to enable them to provide for different kinds of protection. In this way, time/current characteristics have been developed referred to as "inert", "quick", "superquick", and "inert-quick". It is a common requirement for all fuses--be they of any of the specified types--that they shall perform the breaking operation extremely quickly and at a high reliability, if a short-circuit current of high value appears. The continuous extension of the electric energy distribution systems and the increasing value of the distribution voltage led to the value of the desired breaking capacity also being continuously increased. Nowadays it is required that an up-to-date type fuse shall perform the breaking of a 100 kA short-circuit current at a supply voltage of 660 V at a high reliability.
It is a special problem to obtain a suitable shape of the lower section of the time/current characteristic, i.e. the part of it including the small current values. In the prior art, different methods are known to solve this problem. FIGS. 1-3 show three different variants of known fuse-elements 1, 2 and 3. A common feature of them is that zones of diminished cross-section are shaped in the central part of the elements, but the zones are made in different ways. The zones determine the spot where the elements will fuse first in the case of a high overload or short-circuit, and at the same time this is the spot where an arc will arise.
A further common feature of the fuse-elements is that a metal or metal alloy 4 of low melting temperature is applied onto the fuse-element. In case of a small overload, the metal or metal alloy will melt, and in its melted state diffuse into the metal strip increasing its resistance, causing local heating and time-dependent melting. FIGS. 4 and 5 show a fuse-element 5 that is cut in its midst to two pieces, the pieces being connected to each other by an arc-like bent sheet 7 made of the same material as the fuse-element; the connection is established along the edges by the weld 8. Currents of high value cause a melting in the diminished cross-section 6, whereas a smaller overload causes the melting of the weld 8 at the spot of highest temperature where the arc will arise. Sheet 7 serves as heat dissipater delaying the temperature rise.
Curve 10 of FIG. 6 shows the temperature distribution along the fuse-elements 1, 2, and 3 that can be measured inside the casing of the fuse at the highest current value that still fails to result in melting. This curve is characteristic for the steady state during work. At the spot where the diminished--smallest--cross-section is shaped, a slightly higher temperature can be measured (see curve 10) than the one appearing if the cross-section is not diminished (see curve 11). The increase of temperature is only slight, because the conduction of heat is good enough to dissipate the heat arising very quickly, so that only a small temperature difference can develop. In a transient state, however, caused by a short-circuit current of high value, a sudden temperature rise as shown in curve 12, will develop at the smallest cross-section. If this temperature peak value exceeds the melting point of the fuse-element, the element fuses.
In the diminished cross-section a steady state can only occur if the arising heat quantity, being proportional to the square of the current, can proceed in time towards the greater dissipating surfaces. The speed of this heat transmission depends on the temperature difference causing it but it is essential that this difference be of a nature so as not to allow the highest temperature value even to approach the melting point. Conditions are also influenced by the fact that the resistance of the fuse-element increases with temperature. Thus, the value R in the formula I.sup.2.R becomes also a current dependent quantity. It is characteristic for the transient state i.e. in a state when the thermal equilibrum is already upset, that the increase of heat production (a function of the square of the current) is steeper than the increase of heat dissipation (rising according to a linear function). Once the thermal equilibrum is upset, that part of the heat that cannot be dissipated accumulates rapidly, and a very quick temperature rise develops that will finally exceed the melting point and cause the breaking of the fuse.
There is a standard requirement specifying the ratio between the maximum current still failing to cause any melting, and the rated operating current of a given fuse-link. Most of the customers require that a fuse shall, while meeting this requirement, also blow very quickly in the case of a relatively small overload. Generally, the standard regulations prevent the melting of a fuse element during a period of 1 to 3 hours if the current does not exceed 1.3 times its rated value, 1.3.times.I.sub.rated being the lower threshold of the overload range.
In public networks, the standard requirements concerning protection against electric shock can be met more easily, if quickly melting fuses and not inert fuses are used. The melting time of inert fuses is very long and, in the case of long lines of high impedance, the aforesaid standard safety requirements can only be met, if the inert fuse is of a rated current lower than the thermal load limit, i.e. the network cannot be fully made use of. Usually, electricity generating plants allow a maximum melting time of 5 seconds, if the current is two times its rated value. Even quick fuses according to prior art are not apt to meet the 5 second melting requirement if the current is lower than 3.times.I.sub.rated ; other types of fuses perform this duty only if the current exceeds 4-5 times the rated current. It can be seen that a breaking within 5 seconds in the case of an earth leaking current is only possible, if the rated current of the fuse is lower than the load allowed in the given network. And if the fuse is intended to protect a semiconductor, it is due to the excessively long melting time of the fuse that semiconductors must be used, the rated current of which amounts to two or three times the value of the rated current of the equipment they are used in, and consequently such semiconductors are also much more expensive.